1. Field of the Invention
The invention relates to a method for producing a resin pipe for use as the inner lining of existing pipes, and more particularly, to a method for producing a resin pipe which is to be inserted into such existing pipes that have been installed underground or in the building body so as to serve as sewerage pipes or cable ducts, wherein these existing pipes or ducts are repaired or reinforced with the inner lining referred to above.
2. Description of Prior Art
To repair the existing pipes in such a manner as referred to above, there was recently proposed and is widely noticed in the art a kind of lining method ( for example, a method as disclosed on the Japanese Patent Publication Kokai No. 1-295828), in which a hard or semi-hard resin pipe is heated to soften so that it can be inserted in said existing pipes and expanded therein in the radial direction by introducing an amount of hot compressed steam.
In the conventional method, resin pipes made of hard polyvinyl chloride are widely used as the material of the linings. A unit or span length of the existing pipes to be lined varies from about 10 to 100 meters. Accordingly, the resin pipes also have to be of a length corresponding to the unit length.
Such a long resin pipe for the lining process is depressed flat before they are wound up around a take-up drum or bobbin. The wound resin pipe is transported to a site where the lining work is done, and a heating medium such as hot compressed steam is introduced into the resin pipe through one of its opposite longitudinal ends. The thus softened resin pipe is reeled out from the drum or bobbin so that the leading end of the resin pipe is pulled into and through the existing pipe's portion which must be repaired.
In detail, the prior art resin pipes as the lining material have been produced in a manner such that a raw resin is extruded to form a cylindrical pipe having a predetermined diameter, with the cylindrical pipe being heated again to become softer and successively being reeled up on the take-up drum forcibly under a given tension which causes the pipe to become flattened. Alternatively, the extruded pipe is forced through between a pair of depressing rollers disposed in parallel with each other, whereby the intentionally depressed pipe assumes an elliptical shape in its cross section.
The lining resin pipe which is flattened and reeled up in this manner tends to take an almost collapsed configuration having as a whole a greatly reduced thickness in cross section as is shown in FIG. 6. Because this tendency is influenced by any of factors such as a larger diameter of unreeled pipe and a thinner pipe wall, these factors imparted in combination will make worst the state of said reeled-up resin pipe. Such a flat depressed resin pipe reeled on the drum "D" in FIG. 6 is advantageous in that due to a decreased overall volume on said drum an additional r length of the depressed resin pipe 21 can be accommodated on a single drum. However, the collapsed state of the resin pipe is remarkably disadvantageous as will be described below, when the resin pipe is used to carry out the lining process.
Convolutions of the depressed resin pipe 21 shown in FIG. 6 have each in its cross section a pair of bent ends having a relatively small radius of curvature. Merely a small space 22 is formed inside each of the sharply bent ends, and a couple of facing walls 21a and 21a are in close contact with each other between the opposed bent ends, over the entire or most length of the resin pipe. Therefore, the heating medium such as steam cannot be distributed uniformly over the full length of the reeled-up depressed resin pipe 21, when it is heated and softened by forcing the heating medium into the resin pipe through a hose 12 previously connected to the pipe's inner end which was a leading end when the pipe was reeled up. Consequently, it will take not only a longer time to fulfill the heating medium throughout the resin pipe 21, but also the portions thereof in any cross section as well as the portions along the pipe will not be heated and softened uniformly. In other words, the opposite bent ends having the small spaces 22 in cross section will be preferentially heated and softened, while some longitudinal portions will not be sufficiently heated and softened since the heating medium does not flow uniformly through the resin pipe.
As shown in FIG. 5, the outer or leading end of such an ununiformedly heated and softened resin pipe will be secured to a pulling rope 14 so as to be pulled through the existing pipe "P". The portions which have not softened enough will however cause the reeled-up resin pipe to maintain its curved shape, thus necessitating an additional power to overcome an increased resistance to the pulling force, and still making it difficult to insert the resin pipe smoothly into the existing pipe.
The stronger pulling force will bring about the partial or local elongation of the depressed resin pipe 21 at its well softened portions. Due to the forced sliding contact of the existing pipe "P" with the inner surface, the resin pipe will be scratched on its outer surface to produce damages which might break down the resin pipe in the worst case.
In addition to the problems encountered when the resin pipe is inserted, there arise further problems when the internal pressure is charged to expand the depressed resin pipe 21 after it is inserted. Particularly, the inner surface of opposite bent ends and/or the portions having a reduced wall thickness of are likely to produce cracks which might result in burst of the resin pipe in the worst case, because it is softened in an unequal manner, the hair-pin-like bent ends in cross section are apt to excessively deform when the resin pipe expands, its wall thickness has been reduced unevenly due to the elongation when inserted into the existing pipe, and the resin pipe has been scratched on its outer surface.